1. Field of the Invention
The present invention relates to a hydraulic circuit in a device that attaches/detaches a counterweight to/from the rear portion of the body of a work machine such as a hydraulic excavator, a wrecking machine or a crane.
2. Description of the Related Art
At a work machine such as a hydraulic excavator, a wrecking machine or a crane, a work device engaged in excavating operation, crushing operation, loading operation or the like is attached to the front portion of the work machine body and a counterweight is mounted at the rear side of the work machine body so as to balance the weight of the work device. As a given work device is replaced with another work device, the counterweight may need to be switched to a counterweight with optimal weight for the type of operation to be performed by the new work device or the weight of the new work device. Or an additional counterweight may need to be mounted or part of the existing counterweight may need to be detached so as to suit the type of operation to be performed by the new work device or the weight of the new work device. In addition, when the work machine is to be transported to another location, the counterweight will need to be separated from the work machine and moved to the new location to be reassembled at the new work site. In other words, the counterweight needs to be attached to and detached from the work machine body frequently. The counterweight attaching/detaching devices in the related art include, for instance, that disclosed in Japanese Patent No. 2922778.
The counterweight attaching/detaching device disclosed in Japanese Patent No. 2922778 comprises a hydraulic cylinder used to raise/lower a counterweight, which is mounted at the rear portion of the work machine body so as to be allowed to swing up/down, a swing arm attached to the rear portion of a work machine body, which swings up/down as the hydraulic cylinder extends/retracts, and a link member suspended from the front end of the swing arm and connected to the counterweight. When attaching the counterweight to the work machine body, the swing arm is lowered by retracting the hydraulic cylinder, the link member is connected to the counterweight placed on the ground, then the counterweight is lifted to the work machine body by extending the hydraulic cylinder and thus raising the swing arm and finally the counterweight is locked to the work machine body with a bolt.
After the counterweight has been lifted to the work machine body and locked to the work machine body as described above, the hydraulic cylinder is slightly retracted so as to perform work by achieving an unconstrained state in which the hydraulic cylinder, the swing arm and the link member are not subjected to the load attributable to the counterweight. In such an unconstrained state, the force imparted from the counterweight side is not transmitted to the link member, the swing arm or the hydraulic cylinder. The unconstrained state is achieved by slightly retracting the hydraulic cylinder after attaching the counterweight to the work machine body as described above, in order to prevent any damage to crucial parts of the attaching/detaching device, such as breakage of the linking bolt used at the link member, due to vibration transmitted to the counterweight from the body of the work machine during the operation, which would otherwise be transmitted to the link member, the swing arm and the like.
Such a counterweight attaching/detaching device in the related art may include a hydraulic circuit such as that shown in FIG. 7. In FIG. 7, reference numeral 25 indicates a hydraulic cylinder that raises/lowers the counterweight (not shown), reference numerals 50 and 51 respectively indicate a main hydraulic pump and a pilot hydraulic pump both mounted at the work machine body and reference numeral 53 indicates a control valve that is switched via a pilot valve 54. Reference numeral 56 indicates a main duct connecting a bottom chamber 25a at the hydraulic pump 25 and the control valve 53. At the main duct 56, a slow return valve 58, a pressure sensor 70, a stop valve 71, which remains closed as long as the counterweight attaching/detaching device is not engaged in operation, and a pilot-operated check valve 72 are disposed. The pilot-operated check valve 72 prevents the fluid in the bottom chamber 25a from flowing out of the bottom chamber 25a while the hydraulic cylinder 25 is not in operation, whereas it achieves continuity with the hydraulic pressure in a main duct 57 located on the side where a rod chamber 25b is present, applied thereto as a pilot pressure when the hydraulic cylinder 25 retracts.
As described earlier, once the counterweight is locked to the work machine body, the counterweight attaching/detaching device sets the hydraulic cylinder in the unconstrained state by displacing the hydraulic cylinder to a position at which the load of the counterweight is not applied to the cylinder and the like. However, the hydraulic circuit for the counterweight attaching/detaching device in the related art includes the pilot-operated check valve 72 which prevents the counterweight from falling down, disposed so as to block the main duct 56. Thus, an oil chamber sealed with the pilot-operated check valve 72 is formed over the circuit area ranging from the pilot-operated check valve 72 through the bottom chamber 25a. Under such conditions, the hydraulic fluid in the hydraulic cylinder 25 repeatedly contracts and expands as heat is radiated from the engine or the like or as the outside air temperature changes. Then, as thermal expansion of the hydraulic fluid causes the hydraulic cylinder 25 to extend, the hydraulic cylinder 25 can no longer be held in the unconstrained state. Instead, it enters a constrained state in which the counterweight, locked to the work machine body with the bolt, is subjected to an upward force. Under the load of an excessive upward force, crucial parts such as the hydraulic cylinder and the link member may become damaged.
Since thermal expansion of the hydraulic fluid could lead to damage to crucial parts of the counterweight attaching/detaching device, as described above, a pressure sensor 70 is disposed in the main duct 56 connected to the bottom chamber of the hydraulic cylinder 25 in the related art. Namely, as the pressure sensor 70 detects an abnormal pressure, the operator is notified. The operator, having been notified of the abnormal increase in pressure, operates the control valve 53 for the hydraulic cylinder so as to retract the hydraulic cylinder 25, thereby preventing damage to crucial parts such as the hydraulic cylinder 25 and the link. However, since the operator, having been warned of the abnormality via the pressure sensor 70, needs to halt the operation of the work machine and adjust the hydraulic cylinder 25 to the optimal position, work efficiency will be compromised.